The design of user interfaces is becoming a central problem for large, complex systems. User interfaces are the bridge between applications and users, and, as a result, have to cope with the complexities of both. With the advent of new technologies and user-centered concerns, the user interface portion of interactive systems is becoming increasingly large and complex. Faster and more capable machines and networks are providing users with more functionalities and more information, but at the same time overwhelming users (as well as the corresponding processors and networks) with more commands and options presented via the user interface.